The present disclosure is directed generally to a composite antenna structure having the ability to receive or transmit on multiple frequency bands, and more specifically to a composite antenna structure to receive low as well as high frequencies, for example, Digital Video Broadcasting (DVB), analog TV as well as Universal Mobil Communications System (UMTS) and WLAN in different licensed or unlicensed bands.
The receiver antenna represents an important part of any communication system. The antenna dimensions are inverse proportional with the frequency. As the frequency becomes larger the optimal antenna becomes smaller. For multiple communication protocols, spread over various frequency bands, using a single antenna becomes a challenging task.
The trend in the wireless mobile industry is to aggregate multiple communication protocols in a single device. With the new Digital Signal Processors (DSP) capable of executing over 10 billion instructions per second, currently it is possible to run multiple communication protocol on the same platform. In Software Defined Radios (SDR), the same processor is able to run different base band communication protocols. The real challenge is the antenna and the Radio Frequency (RF) front end. Each communication protocol requires a different antenna and a different RF front end raising cost and real estate issues.
One composite antenna structure which operates in multiple frequency bands is disclosed in U.S. patent application Ser. No. 10/859,169 filed Jun. 3, 2004 for “Modified Printed Dipole Antennas For Wireless Multi-band Communication System” by Emanoil Surducan, Daniel Iancu, and John Glossner. Those multi-band antennas are designed for WLAN dual frequency bands of, approximately 2.4 GHz and 5.2 GHz, and GSM and for 3G multi-band wireless communication devices, of approximately 0.824-0.960 GHz, 1.710-1.990 GHz and 1.885-2.200 GHz.
The effort to miniaturize the antenna and add more frequency bands to an existing antenna structure is not trivial. As the dielectric constant increases, the antenna will concentrate more energy resulting in less bandwidth and lower efficiency. To add more frequency bands meta materials are used on the ground plain in a periodic structure as reported in Alexander A. Zharov, Ilya V. Shadrivov, and Yuri S. Kivshar “Nonlinear Properties of left-handed Metamaterials,” Phys. Rev. Lett. 18, July 2003, 91:3, pp 37401-1 to 4. A combination of composite magneto-dielectric substrate is used to increase the bandwidth and efficiency as described in Hosein Mosallaci, Kamal Sarabandi, “Engineered meta-substrates for antenna miniaturization”—Proceeding of URSI EMTS 2004, Vol. 1, pp. 191-193, Pisa, Italy.
An antenna with low reluctance material positioned to influence radiation pattern is described in U.S. Pat. No. 5,982,335. An antenna system with active spatial filtering surface is described in U.S. Pat. No. 6,806,843. A planar receiving TV antenna with broadband is described in U.S. Pat. No. 4,860,019.
The miniature micro-strip composite multi-band antenna of the present disclosure allows the reception of various signals in different frequency bands with a single antenna. The composite antenna is shown as constructed as a microstrip dipole antenna with a shorted antenna placed in the near field. The technique is applicable for communication protocols at any frequencies. As an example, the present multi-band antenna structure adds more received frequency bands to the UMCS bands and, especially at low frequencies, 100 to 1000 MHz. It also exhibits increased gain in these bands.
The multi-band antenna structure includes a first antenna having a first band width about a first middle frequency and a second antenna spaced and electrically isolated from the first antenna. Ends of the second antenna are shorted to each other and the second antenna floats electrically. The first and second antennas are planar and superimposed in parallel planes. At least first and second layers of dielectric material of a first and second thickness respectively are between the two antennas. A third layer of dielectric material of a third thickness is between the two antennas.
The total thickness of the three layers is less than the quarter wave length of the lowest middle frequency. The first thickness of the first layer adjacent the first antenna is greater than the third thickness of the third layer adjacent the second antenna, and the first and third layers have the same permittivity. The second thickness of the second layer between the first and third layers is greater than the first thickness of the first layer, and the second layer has a low permittivity than the first and third layers.
The antenna structure has at least one band with a middle frequency below 1 gigahertz and an S11 of less than −10 dB and VSWR of less than 2. Alternatively, the antenna structure has at least one band with a middle frequency below 2 gigahertz and an S11 of less than −10 dB and VSWR of less than 2, and at least two band with a middle frequency above 2 gigahertz and an S11 of less than −10 dB and VSWR of less than 2.
The first antenna may include a matching circuit connected between a feed terminal, a ground terminal and the antenna. The antenna structure may include an electrically floating split ring resonator spaced from the first and second antennas. The second antenna and the resonator are in a common plane which is parallel to a plane of the first antenna.
These and other aspects of the present disclosure will become apparent from the following detailed description of the disclosure, when considered in conjunction with accompanying drawings.